ONITSHA, Nigeria (Reuters) - Nigeria’s immigration service has launched an investigation after two of its staff members were arrested on suspicion of trying to traffic girls out of the country, it said on Friday.

Thousands of girls are each year taken illegally out of Africa’s most populous country, where 70 percent of the 190 million inhabitants live on less than two dollars a day. A large proportion of them arrive in Europe.

“We received information that two of our officers were arrested yesterday at Murtala Muhammed International Airport, Lagos, while trying to facilitate the trafficking of some underaged girls out of the country,” spokesman James Sunday said.

“The comptroller general of immigration has been briefed about this and we have started investigations,” he added.

The spokesman did not say how many girls were involved, nor did he give their ages or say where they were allegedly being taken. The movement of people out of the country by criminal gangs, often by sea rather than air, has become a major problem for authorities in the west African country.

The British government says Nigeria is the fourth largest source of human trafficking to its shores and Nigerians are the largest national group among African migrants travelling to Libya and trying to cross from there to Italy in boats.

A report by the International Organisation for Migration (IOM) published last year estimated that 80 percent of Nigerian girls arriving in Italy by sea might be trafficking victims.